Dielectric guide for electromagnetic waves



Aug. ze, 1958 A G FOX DIELECTRIC GUIDE FOR ELCTROMAGNETIG WAVES FiledAug. 18. 1954 v. ...Hl

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/NVENTOR A. G. FOX 5v/@7% yf ATTORNEY United States Patent OllceDIELECTRIC GUIDE FR ELECTRM'AGNETIC WAVES .Arthur G. Fox,.RumsonN. J.,assigner to Bell Telephone Laboratories, Incorporated, New York, acorporation of NewYork Application August13,;1954, Serial No. 450,626

6 Claims. (Cl. 3331-495) .This invention relates to microwavetransmission systems andA more particularlytothe.transmissionofelectromagnetic wave energy having wavelengths ofseveral millimetersalong dielectric transmission lines having no conductive shields.

In applicants copending application, Serial No. 274,313, filed yMarch 1,1952, now U. S. Patent 2,794,959, issued I une '4, `1957, itis taughtthat electromagnetic wave energy may be guided along a transmissionmedium consisting solely of dielectric material, in other words, anall-dielectric media as opposed to the vmore well-known transmissionmedia of thetypes either in which a longitudinal conductive shield isplaced to surround the dielectric material or in which a conductiveaxial core is provided within the dielectric material. Investigation hasindicated Vthat the guiding eiect is retained when using a very thindielectric rod only a fraction of a wavelength in diameter. A greatportion lof the energy when launched upon such a rod in the proper modeis conveyed in a eld surrounding the dielectric material and thereforeis not subject to its losses. For'this reason the transmissionattenuation of a thin all-dielectric guide may be made verylow. However,transmission ,applications ofthese'guides have been substantiallylimited to experimental installations because of the physical Weaknessof the material forming the guide. Not only is it subject to fracture bysharp bends-or kinks or by sudden changes of strain, but even aconstantly applied force, for example, due to the weight of aself-supported `vertical run of the guide, tends to stretch and deformthe guide. This deformation may increase mode degeneration and radiationlosses. Rigid `mountings for supporting the guide will overcome thislimitation to some extentbut such supports destroy the flexibility andsimplicity'of the unshielded transmission system which are among itsmost useful attributes.

It is therefore an object of the invention to add strength to anunshielded dielectric transmission media while maintaining itsyflexibility and simplicity.

It is another object of the invention to transmit afdesired mode ofpropagation along an unshielded transmission medium which medium vat thesame time dissipates and prevents transmission of undesired modes ofpropagation.

These objectsare accomplished inthe-embodiment to be describedhereinafter by transmitting awave energy in a mode, the field patternofwhich resembles the dominant transverse electric mode in aconductively shielded rectangular wave guide, along an unshieldeddielectric guide having a central metallic core of resistive material.Since the field of this wave does not tend to induce longitudinalcurrent components running along the core, the wave energy is propagatedsubstantially unalectediby'tlaepresence of the resistive core. The metalCornhover, adds substantial mechanical strength to `the guide. On theother hand, `a predominant spurious'wave into Which/the desired wavetends to `degenerateis ltransverse magnetic, with a field pattern`resembling the `dominant vmode of coaxial transmission lines. Thisspurious wave produces llifatented Allg. 26, 1.9.5.8

longitudinal currents in theresistivercore that cause dissipation of theWave energy.

These and other objects, the nature of the present invention and itsladvantages and features, will `appear more fully upon considerationofthe several illustrative embodiments now to' be described in connectionWith the accompanying drawings Vin which:

Fig. 1 is a pictorial representation of a millimeter wavelengthmicrowave system in which two electromagnetic Wave devices areinterconnected along a curving path by a transmission line having aresistive core in accordance with the invention;

Fig. 2 represents an alternative cross section for the transmission lineof Fig. 1;

Figs. 3 and 3A represent the electric ield pattern of the desired modeoftransmission on a transmission line of the-type employedV by thepresentinventiom and Figs. 4 and 4A represent the electriceld pattern ofa spurious wave which is dissipated by the transmission line ofthepresent invention.

Fig. 1 illustrates'how a dielectric guide in accordance with theinvention is used to connect two electromagnetic wave devices 12 and '13`which may be a source vand a load, respectively. This connection may beone that requires a curving path and perhaps some freedom of movementbetween Idevices '12 and y13. At the lower frequencies sucha connectionmight have been made by a conventional coaxial line or by one of thewell-known corrugated or vertebratypes of wave guide connections, butinasmuch as the present apparatus is lcontemplated as operating inthewavelength range vof several millimeters, these well-known connectorslare not satisfactory. In accordance Vwith thepresent invention,therefore, this connection is made by an elongated member 14 ofdielectric material, having a metallic core 15 of high resistancematerial extending substantially the length ,of Ymember 14, andhavingnoexternal conductive shield.

The body of-member=14 is made of a nonconductive material having adielectric constant substantially different vfrom the atmosphere.surroundingit which may be air, any other gas or vacuum, and thereforehaving a phase `velocity for wave energy substantially dilerent from thephase velocity of wave energy in that atmosphere. By Way ofexample,-.the synthetic plastic materials, polystyrene, polyethylene,Teflon and laminated polyex, have proved satisfactory, to .mention onlyseveral specic materials.

The transverse cross 'sectionof member14 is ovoid, i. e., isprovidedvwith different orthogonal dimensions in any given cross sectionso that the phase velocity of dominant waveenergy polarized parallel toone of these dimensions is substantiallyditferent from'the phasevelocity of dominant wave energy polarized orthogonally'thereto. It hasbeen found that this sort-of cross section maintainsthe fpolarization ofthe =wave launched upon it and substantiallyfaidsin reducing Vthetendency for it to degenerate into other modes as yfully described inthe above-mentioned copending application. As illustrated by way ofspecific example in Fig. l, member 14 is of rectangular cross sectionhaving a longer transverse dimension of several times Ithe Ashortervtransverse dimension thereof. .Howeven oblong or elliptical crosssections, such as the cross section of guide 30 represented in thealternative.crossfsectional view of "Fig, 2, may be usedif thelengthsof'the`.major axisvare diterent. Long slender guidesfofutheflatter cross .sections are -easier to make by presently knowmmethods ofmanufacture such as by extruding.

Core 15 comprisesan axiallyimbedded Wire-like member that is surroundedby or circumferentially sheathed within the dielectric and :runssubstantially the entire length of member .14. -Core 15 can be,characterized as a dissipative conductor" and is made of a highresistance material. For the purposes of the present invention, theordinarily relative term high resistance will be taken specifically tomean the resistance of a material having a resistivity of greater thanabout l 10-6 ohm centimeters at degrees centigrade. Within this categoryare the commercially available resistance wires commonly used as heatingelements in electrical appliances, such as constantan, Nichrome andClimax wire. Also within this category are the conductive materialsbismuth, bronze, copper-manganese, gallium, German silver, Monel metal,osmium, and most of the alloy steels. Opposed to this group of highresistance materials are the low resistance materials commonlyrecognized as being good conductors of electrical energy havingresistivities below l5 106 ohm centimeters at 20 degrees centigrade.This group includes such materials as copper, brass, aluminum, gold,silver, cadmium, chromium, iron, molybdenum and zinc.

To couple the Wave energy frorn device 12 and to launch it in the propermode upon guide 14, a transducer of conductively bounded components isemployed. This transducer comprises a rectangular wave guide 16 whichhas one end coupled to device 12 according to conventional practice sothat a dominant transverse electric or TEN mode is excited within guide16. The other end of guide 16 is ared out into a rectangular horn 17which has its wide and narrow mouth dimensions extending parallel to thewide and narrow dimensions of guide 16 respectively. One end of guide 14is pushed through the horn to extend several wavelengths into guide 16.The match between guide 14 and guide 16 is improved by providing a taper18, extending along several wavelengths of the portion of guide 14within guide 16. A similar arrangement comprising horn 19 andrectangular guide 20, which may be identical to horn 17 and guide 16,respectively, is provided at the other end of guide 14 to couple it todevice 13. Thus the dominant TEM, mode in guide 16 is launched as adominant hybrid wave upon dielectric guide 14. This wave is the lowestorder hybrid electromagnetic wave that may be supported by thedielectric guide and that has an electric field pattern which closelyresembles the eld pattern of the dominant TE wave in a hollow metal tubewave guide. The electric field pattern of such a wave is shown in Figs.3 and 3A to be discussed in detail hereinafter. For the purposes of thepresent specification, this wave will be referred to as the dominanthybrid electromagnetic mode which will be abbreviated as the HEM1 mode.As the crosssectional dimensions of the dielectric guide are reduced, amajor portion of the wave power of this mode will be forced into thesurrounding space. In order to keep the attenuation low for this mode,therefore, it is preferable to use a very slender dielectric member forguide 14 such that the maximum cross-sectional dimension thereof is asmall fraction of a wavelength of the highest frequency to betransmitted.

Any discontinuity along guide 14 produces a tendency for the HEM1 modeto couple with and degenerate into another mode on guide 14. This newmode resembles both the mode in a coaxial transmission line and the TMmmode in a hollow round metal wave guide and has predominantly transversemagnetic lines of force and radial electric lines of force in a givencross section. This field pattern is more fully disclosed hereinafter inconnection with Figs. 4 and 4A. For the purposes of this specification,this new mode will be referred to as the lowest order transversemagnetic mode or TM1 mode. Its nature is fully considered in a paperentitled Singleconductor surface-Wave transmission lines, by GeorgGoubau in the Proceedings of the Institute of Radio Engineers, volume39, No. 6, June 1951, pages 6l9624.

Referring therefore to Figs. 3 and 3A, and 4 and 4A, the electricalfield patterns of the HEM1 and TM1 modes are illustrated, respectively.To simplify the illustration of the field patterns and the comparison tobe made between them, the transmission medium of Figs. 3 and 4 isillustrated as having a dielectric portion 31 and a resistive core 32which are round in cross section, even though as noted above it ispreferable to employ an ovoid cross section. However, the eld pattern onthe ovoid cross sections docs not differ qualitatively from those shownin Figs. 3, 3A, 4 and 4A.

From Figs. 3 and 3A it will be seen that the electric eld of the HEM1mode forms closed loops that lie substantially in curved surfaces thatpass in the vicinity of the dielectric portion 31. These surfaces aresubstantially normal to the same plane through the axis of the linc(represented at A on Fig. 3) and tend to fan out in the space farthestaway from the axis of the line. In the vicinity of core 32, these loopshave no component parallel with the longitudinal axis of the core, butrather extend transversely across the core. Thus no longitudinal currentcomponents are produced to flow along core 32. This particular mode isguided only by the dielectric portion 31 with substantially nointerference from core 32. Measurements have indicated that the presenceof thc core perturbs the HEMI mode only slightly and adds but verylittle to its electric losses. Thus the presence of the core addsphysical strength to the dielectric guide, allows the guide to be morenearly self-supporting, and allows it to be bent smoothly and with readyflexibility around gradual curves.

From Figs. 4 and 4A it will be seen that the electric held of the TM1mode forms radially extending loops each of which terminate upon thecore 32. ln order to complete the continuity of each loop, longitudinalreturn currents must flow through core 32. These currents causedissipation of energy by the resistive material of the core with theresult that the TM1 mode and all other modes similarly producingsubstantial longitudinal currents are rapidly attenuated. Since it is afundamental of coupled transmission lines that one wave will not easilycouple with or degenerate into a second wave which is highly attenuated,the tendency of the HEMl mode to degenerate into the TM1 mode issubstantially reduced.

As used in the present specification and claims, the term shiel isrestricted to mean the conductive boundary of a wave transmission paththat actually forms a part of the propagation media and substantiallyinfluences the propagated wave. In this sense it includes the conductiveportion of the conventional hollow pipe wave guide. Therefore, the termsdielectric wave guide or unshielded guide are intended to mean a rod orcolumn of material having a dierent and generally a greater dielectricconstant than its immediate surroundings not closely surrounded by aconductive shield. However, it is understood that these terms are notintended to exclude structures which are provided with a boundary,conductive or otherwise, located at a suflicient distance from theboundary between the high and low dielectric constant media to havelittle or no eiect on the propagation of the desired wave along thedielectric portion of the guide.

In all cases, it is understood that the above-described arrangements aresimply illustrative of the many possible specific embodiments which canrepresent applications of the invention. Numerous and varied otherarrangements can be readily devised in accordance with said principlesby those skilled in the art without departing from thc spirit and scopeof the invention.

What is claimed is:

l. In combination, first and second electro-magnetic wave devices, anelongated member of dielectric material connecting said devices, saidmember being unsheathed whereby a substantial portion of wave power isconveyed in a field surrounding said member, and a metallic core of highresistance material extending substantially the length of said member todissipate wave energy which produces longitudinal currents in said core.

2. In an electromagnetic wave transmission system, a Waveguidng pathcomprising a Wire-like element enclosed Within a sheath of mrtterialhaving a high dielectric constant, and means at each end of said pathfor launching an electromagnetic Wave upon said path in an electric eldpattern having continuous loops which extend transversely through saidsheath and through and around said wire, said wire being resistive tosubstantially dissipate wave energy in electrical field patterns havingloops which terminate upon said wire.

3. In combination, first and second electromagnetic Wave devices, anelongated member of nonconductive material connecting said devices, saidmember having diierent orthogonal transverse dimensions in any givencross section and being unsheathed with the outside surface thereofexposed to the atmosphere surrounding said member and having adielectric constant substantially different from that of saidatmosphere, means interposed between an end of said member and each ofsaid devices for launching an electromagnetic wave upon said member inan electric eld pattern having continuous loops which extendtransversely through said member, a metallic wire-like element extendingthrough substantially the center of the cross section of said member,said Wire being highly resistive to substantially dissipate Wave energyhaving tield patterns that are dierent from the eld patterns launched bysaid means in that said different patterns have electric eld loops whichterminate upon said wire.

4. In combination, first and second electro-magnetic wave devices, anelongated member of dielectric material connecting said devices, meansfor coupling one of said devices to said member to 'aunch waves thereinin the hybrid electromagnetic mode, means coupling the other of saiddevices to said member to abstract energy there from, and meansextending along said member to couple to and dissipate Waves propagatingtherein the transverse magnetic mode.

5. In an electromagnetic Wave transmission system, a source ofelectromagnetic energy, an elongated member of dielectric material,means for launching Waves from said source for propagation along saidmember in the hybrid electromagnetic mode, and means extending alongsaid member to couple with and dissipate electromagnetic Waves of thetransverse magnetic mode.

6. In an electromagnetic wave transmission system, a source ofelectromagnetic energy, an elongated member of dielectric material,means for launching Waves from said source for propagation along saidmember in the hybrid electromagnetic mode, and a strength member ofdissipative material positioned in and extending along said dielectricmember to couple with and dissipate waves traveling along saiddielectric member in the transverse magnetic mode.

Peters Oct. 27, 1936 Crise Apr. 10, 1951

